They allow direct installs of add-ons that they know were tested properly and work fine on a phone, not those where they can't guarantee that level of stability (while still giving you that as an option if you feel like experimenting).
I don't see how that's arbitrary, I see it as a well thought out process, even if I wish more add-ons were added to that collection.
I agree that there should be more allowed add-ons. Engineering didn't put in all the effort to implement the add-ons APIs on Mobile only for it to be restricted to such a small set. Unfortunately that's a product decision.
